Detection of Estrus in Mares Back »

A critical factor in the success of a breeding operation is determining the correct time to breed a mare.  Mares have reproductive cycles that average 21 days in length.  The adjective estrous is used to describe these cycles.  The length of the estrous cycle in a mare is 21 days.  As the mare develops a large dominant follicle she will exhibit signs of “heat”, or “estrus” which is when she is receptive to being bred by a stallion.

Stallions can be very useful in determining when a mare is in estrus. If a mare is nearing the time of ovulation she will demonstrate behaviors such as:

  • Winking (clitoral)
  • Frequent urination
  • Excitement
  • Posturing (assuming a stance to be bred)
  • Raised tail
  • Seeking the stallion
  • Allowing for mounting to occur*

Anestrus is the term to indicate that are mare is not in heat.  “The mare did not display behaviors of estrus, she is anestrus.”  Behaviors of anestrus include:

  • Restless in presence of stallion
  • Striking
  • Squealing
  • Kicking
  • Pinning ears
  • Clamped tail
  • Ignores stallion or is aggressive towards stallion
  • Will not allow mounting*

The single most important factor determining if a mare is in estrus and ready to be bred is whether or not she will stand to be mounted and mated by the stallion. Sometimes a mare may show many signs that she in nearing estrus, but may not quite be ready to be bred.

Mares are scored on a 5 point system generally from 0 to 4 with a score of 0 given to a mare who is aggressive towards a stallion and a score of 4 given to mares who are ready to stand for mating.

Behavior score (5 point system)

Aggressive towards stallion 0
Indifferent 1
  2
Mild signs of estrus 3
Willing to be mounted 4

On average estrus last 5 to 7 days.  Some mares may exhibit signs of estrus for shorter or longer periods of time. Because mares ovulation just before the end of estrus it is important to be diligent with detection of estrus and records.  Generally, it is recommended to breed your mare on the third day of estrus and then every other day until she is no longer exhibiting signs of estrus.  However, if you have a mare who shows signs of estrus for only 3 days on average, you may choose to breed her sooner.


For more information on detecting signs of estrus in mares, or methods for estrous detection please refer to SDSU Extension publication Detection of Estrus in Mares.

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